Each email or Usenet message has a unique Message-ID. These are indexed at various providers which archive mailing lists, with Gmane being the most notable in the Free Software community.
If you right-click on the Message-ID of a message in Thunderbird, you can choose to "Open Browser with Message-ID". By default this menu item opens sensible-browser with a Google Groups page, which, while indexing of Usenet, remains ignorant about most mailing lists. Most people reading this will probably find that Gmane carries their favourite mailing lists, while Google does not.
This is configurable, but sadly you have to choose one or another. In Thunderbird, go to Edit → Preferences → Advanced → Config Editor, and set the "
mailnews.messageid_browser.url" property to
http://mid.gmane.org/%mid. (default of